Projectors are used for business, education, diagnostics, entertainment and other purposes to project images from an image source onto a screen or the like for viewing. Many different types of projectors are known, some examples being slide projectors, movie projectors, overhead projectors, and so forth. In some projectors an image which is fixed on a film, slide, or the like, is projected onto a viewing screen. In another type of projector an image is developed or formed in a medium, such as a liquid crystal device, and the image is projected onto a viewing screen; sometimes these are referred to as liquid crystal projectors.
One example of liquid crystal projector uses a twisted nematic liquid crystal cell to modulate light from a light source to produce an image for projection. In such a projector polarized light is selectively transmitted or blocked by the cooperative relation between a twisted nematic liquid crystal cell and an optical polarizer.
Another example of liquid crystal projector uses the principle of selective scattering or transmitting of light by a liquid crystal device in cooperation with an optical aperture and stop to discriminate between transmitted and scattered light to provide an image for projection. A liquid crystal device sometimes referred to as NCAP, PDLC, LCPC and possibly by other names is useful to provide the selective light scattering or transmitting in such a projector. One example of such a projector is disclosed in U.S. Pat. No. 4,613,207. Several examples of liquid crystal materials or devices useful in such a projector are disclosed in U.S. Pat. Nos. 4,435,047, 4,606,611, 4,688,900, and 4,728,547. For brevity, such materials and devices which are operable to scatter light or to transmit light collectively may be referred to below as NCAP materials or NCAP devices, and liquid crystal projectors using NCAP materials collectively may be referred to below as an NCAP based LCD projector (“LCD” being a conventional shorthand for liquid crystal display or liquid crystal device) or more simply as NCAP projector.
NCAP projectors which use the switchable light scattering properties of NCAP materials to modulate light have been demonstrated for example, as is disclosed in U.S. Pat. No. 4,613,207. In one embodiment of a projector disclosed in the '207 patent the light source is focused onto a small aperture located in a plate or mask between the projection lens and an NCAP device which is operable as a display to form an image. If the NCAP device (or a picture element, pixel or other part thereof) is in a clear (non-scattering light transmitting) state, substantially all of the light from the NCAP device (or from the indicated pixel) passes through the aperture and is collected and projected by the projection lens. Such transmitted light sometimes is referred to as specular light or specularly transmitted light. Light scattered by the NCAP device or from one or more pixels thereof (sometimes referred to as scattered light) is redirected away from the aperture and is blocked or stopped by the plate in which the aperture is located. In such a projector some of the scattered light may also impinge on and pass through the aperture; this light leakage can reduce contrast of the output. It would be desirable to improve contrast, for example, to increase the contrast or contrast ratio between bright and dark areas of a projected image.
Another embodiment of NCAP projector disclosed in the above '207 patent uses Schlieren optics. A projector which uses Schlieren optics may be referred to as a Schlieren projector, Schlieren optics projector or dark field projector. The Schlieren optics discriminate between light that is transmitted and light that is scattered by a display; the Schlieren projector projects scattered light and blocks transmitted light. An advantage of a Schlieren projector is an improved dark field, which can lead to higher contrast. One reason for such contrast improvement is improved discrimination between the light which is scattered by the NCAP device and the light which is transmitted by that device (specular light); the scattered light is collected and used in the output image, but the transmitted light (specular light) is blocked by a mask or stop in the light path and, therefore, does not reduce contrast.
Various display devices have been used as a “light engine” to create an image for projection, for example using various of the above-mentioned projectors. For example, in U.S. Pat. No. 5,519,524 is disclosed a miniature image source in which electrical input is provided by an electrical drive which includes respective electrodes and transistors associated therewith in an active matrix type liquid crystal display. The electrodes and transistors of the active matrix display are formed in/on a semiconductor substrate, sometimes referred to as an active matrix or a TFT (thin film transistor) device or as an active matrix substrate; one or more counter electrodes are opposite the respective active matrix electrodes. Respective picture elements are formed by the counter electrode and respective electrodes and the liquid crystal material therebetween. Alignment of liquid crystal material between respective active matrix electrodes and counter electrode(s) can be affected by providing electric field between the electrodes; and characteristics of polarized light, e.g., the direction of polarization, transmitted through the miniature image source can be changed or not depending on whether or not a field is applied and the magnitude of the field. Another example of an active matrix (or TFT) type of display device is disclosed in U.S. Pat. No. 5,532,854.
Advantages of size, power, manufacturing, resolution, and so forth, inure to active matrix (TFT) type display systems. Such systems are referred to as electronic input type display systems. Another type of electronic input type display system is referred to as MIM. Others also may exist, may be developed in the future and may be useful in the invention.
In electronic input type display systems it is necessary to provide space between respective active matrix transistors and electrodes directly associated therewith, thus, providing space between respective relatively adjacent picture elements. Such space provides electrical insulation or isolation and is an area in which conductors or conductive paths may be located to connect to respective transistors or other parts of the device. Such space usually is not active to present changeable image characteristics of the optical output of an active matrix display device and may be referred to as optically dead space.
In a reflective electronic type display system there usually is some type of reflector or reflective surface or material to reflect light. For example, electrodes may be reflective or include a reflective coating or treatment and/or a separate reflector may be provided, as is known. Sometimes light which is incident on the optical dead space also is reflected. Thus, light incident on such reflector is reflected back in the opposite direction; if the incident light impinges on the reflective surface at an angle other than normal, the angle of incidence will be the same or substantially the same as the angle of reflection as is the case for conventional reflectors and if the incident light is incident normal to the reflector. Where the light is transmitted through liquid crystal material the light may or may not have changed polarization characteristics, depending on the current alignment of the liquid crystal material; but where the light is incident on optically dead space and reflects back from that space, the polarization characteristics of the light usually would not be altered as a function of electrical input, as there is no electrode there to apply electric field. Therefore, the existence of the optically dead space may have a detrimental affect on the resolution and/or contrast of the optical output.
A scattering type liquid crystal system may include a liquid crystal display, such as NCAP material or an NCAP display, and a drive device, such as an electronic drive, for example, an active matrix, thin film transistor (TFT), metal insulator metal (MIM) or some other drive device for the display. In a reflective type of NCAP display system, optically reflective portions reflect light passed through the display device back through the display device for output as scattered or as unscattered (transmitted) light. It has been discovered that due to scattering caused by portions of the liquid crystal display device which are aligned with the optically dead spaces or with certain components or elements in the optically dead spaces of the drive unwanted scattered light may be projected by a Schlieren optics projector. Such scattered light may bypass the stop so as to be projected, which reduces the contrast or contrast ratio of the projected image. Accordingly, it would be desirable to eliminate from projection scattered light which does not derive from intended picture elements or areas of the display.